Variable gain amplifier circuit and filter circuit

ABSTRACT

This variable gain amplifier is provided with an operational amplifier. The non-inversion input terminal of the operational amplifier is connected to a reference potential. A feedback resistor is connected between the output terminal and inversion input terminal of the operational amplifier. An input resistor is inserted between the inversion input terminal of the operational amplifier and the input terminal of the variable gain amplifier circuit. An adjustment resistor is connected between the inversion input terminal of the operational amplifier and the reference potential. The resistance value of the adjustment resistor is controlled in such a way as to maintain constant against the resistance value change a combined resistance value in its parallel connection with the input resistor when changing the resistance value of the input resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/987,619,filed on Dec. 3, 2007, now U.S. Pat. No. 7,679,447, which claimspriority to Japanese Patent Application No. 2007-036165, filed Feb. 16,2007, which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the amplification technology ofelectric signals, and more particularly to the technology of anamplifier circuit whose gain can be varied.

2. Description of the Related Art

FIG. 1 is explained. FIG. 1 is one example of a variable gain amplifiercircuit whose gain can be changed.

In FIG. 1, the non-inversion input terminal of an operational amplifierOP1 is connected to the ground (reference potential, virtual earth) ofthe circuit. A feedback resistor R0 is connected between the outputterminal and inversion terminal of the operational amplifier.

An input resistor R1 is connected between the input terminal IN of thisvariable gain amplifier circuit and the inversion input terminal of theoperational amplifier OP1. The resistance value of this input resistorR1 can be changed.

An amplifier circuit with a configuration such as that shown in FIG. 1is a typical inversion amplifier circuit. It is widely known that thegain G of the inversion amplifier circuit is as follows (here, noinversion of a signal is considered):G=V ₀ /V ₁ =R0/R1In this case, when changing the value of the input resistor R1, the gainof the circuit varies. FIG. 2 is a graph showing the change of thefrequency and phase characteristics of the circuit shown in FIG. 1 whenchanging its gain thusly.

As seen from FIG. 2, when reducing the gain of the circuit shown in FIG.1 by increasing the value of the input resistor R1, the phasecharacteristic also varies and frequency at which a phase slews shiftstowards the higher frequency side. This is because the amount offeedback from the output of the operational amplifier OP1 to theinversion input terminal is changed by increasing the value of theresistor R1. Such a phase characteristic change can cause distortion inthe waveform of a signal amplified by the circuit.

In the circuit shown in FIG. 1, the amount of feedback from theoperational amplifier OP1 to the inversion input terminal is as follows:V₀×R1/(R1+R0)

Concerning the present invention, for example, Japanese PatentApplication No. H8-116224 discloses a technology for reducing gain errorthat is due to the conduction resistor for a gain changing-over switchand reducing the total resistance value of resistors used for creatinggain change by making the ratio between the input resistor and theconduction resistor for a dummy switch that is connected to it in seriesand is in the normal conduction state almost the same as the ratiobetween a gain setting resistor, which is a feedback resistor, and theconduction resistor for a gain changing-over switch in the inversion ornon-inversion amplifier that uses an operational amplifier.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a variable gainamplifier circuit in which the amount of feedback does not vary evenwhen changing gain to use it in various circuits.

A variable gain amplifier circuit in one aspect of the present inventioncan vary its gain. The variable gain amplifier circuit comprises anoperational amplifier whose non-inversion input terminal is connected toa reference potential, a feedback resistor that is connected between anoutput terminal and an inversion input terminal of the operationalamplifier, an input resistor that is inserted between the inversioninput terminal of the operational amplifier and an input terminal of thevariable gain amplifier circuit, and an adjustment resistor that isconnected between the inversion input terminal of the operationalamplifier and the reference potential and whose resistance value iscontrolled in such a way as to maintain constant against the resistancevalue change the ratio between a combined resistance value in a parallelconnection between the input resistor and the adjustment resistor andthe resistance value of the feedback resistor when changing oneresistance value of the input resistor and the feedback resistor.

According to this configuration, even when changing one resistance valueof the input resistor and feedback resistor in order to change the gainof the circuit, the change in the amount of feedback of the operationalamplifier can be suppressed to maintain it constantly. Therefore, evenwhen changing the gain of the circuit, its phase characteristic does notvary.

Alternatively, the above-described variable gain amplifier of thepresent invention can be composed in such a way that a resistance valueof the feedback resistor may be fixed, that a resistance value of theinput resistor may be variable, and that a resistance value of theadjustment resistor may also be controlled in such a way as to maintaina constant combined resistance value in the parallel connection betweenthe input resistor and the adjustment resistor against the gain changewhen changing the resistance value of the input resistor.

According to this configuration, even when changing the resistance valueof the input resistor in order to change the gain of the circuit,changes in the amount of feedback of the operational amplifier can besuppressed in order to maintain it constantly. Therefore, even whenchanging the gain of the circuit, its phase characteristic will notvary.

In this case, the input resistor can also comprise a plurality of firstresistors with respective different resistance values and a firstselector switch that selects one of the first resistors and connects itbetween the inversion input terminal of the operational amplifier andthe input of the variable gain amplifier circuit, and the adjustmentresistor can also comprise a plurality of second resistors eachcorresponding one-to-one to each of the plurality of first resistors andin which a combined resistance value obtained when a combination of thecorresponding first and second resistors are connected in parallel hasthe same resistance value in all corresponding combinations, and asecond selector switch that selects a second resistor corresponding tothe first resistor selected by the first switch from the plurality ofsecond resistors and connects it between the inversion input terminal ofthe operational amplifier and the reference potential.

According to this configuration, when changing the resistance value ofthe input resistor, the resistance value of the adjustment resistor canbe controlled in such a way as to maintain constant against theresistance value change the combined resistance value in its parallelconnection with the input resistor.

In this case, alternatively, the first selector switch can also becomposed of transistors each of which is connected to the first resistorin series, and the second selector switch can also be composed oftransistors each of which is connected to the second resistor in series.

Alternatively, the above-described variable gain amplifier of thepresent invention can be composed in such a way as to perform controlfor the purpose of changing one resistance value of the input resistorand the feedback resistor on the basis of the signal level of an outputsignal of the variable gain amplifier circuit.

According to this configuration, an automatic gain control (AGC) circuitwhose phase characteristic does not vary even when changing the gain ofthe circuit can be provided.

Alternatively, the above-described variable gain amplifier of thepresent invention can be composed in such a way that the resistancevalue of the feedback resistor may be variable, that of the inputresistor may be fixed, and that of the adjustment resistor may also becontrolled in such a way as to maintain a constant ratio between thecombined resistance value in the parallel connection between the inputresistor and the adjustment resistor and the resistance value of thefeedback resistor when changing the resistance value of the feedbackresistor.

According to this configuration, even when changing the resistance valueof the feedback resistor in order to change the gain of the circuit,changes in the amount of feedback of the operational amplifier aresuppressed to maintain a constant feedback level. Therefore, even whenchanging the gain of the circuit, the phase characteristic of thecircuit does not vary.

A filter circuit in another aspect of the present invention can vary again of the filter circuit. The variable gain amplifier circuitcomprises an RC active filter composed of an operational amplifier, aresistor and a capacitor, an input resistor that is inserted between aninput terminal of the RC active filter and an inversion input terminalof the operational amplifier, and an adjustment resistor that isconnected between an inversion input terminal of the operationalamplifier and a reference potential of the filter circuit. A resistancevalue of the adjustment resistor can be controlled in such a way as tomaintain constant a parallel resistance value between the adjustmentresistor and the input resistor when changing a resistance value of theinput resistor.

According to this configuration, even when changing the resistance valueof the input resistor in order to change the DC gain of a circuit, thechange in the amount of feedback of the operational amplifier can besuppressed to maintain it constantly. Therefore, even when changing theDC gain of the circuit by changing the resistance value of the inputresistor, the cut-off frequency and Q value of the filter can beprevented from changing.

The variable gain amplifier circuit in another aspect of the presentinvention can vary its gain. The variable gain amplifier circuitcomprises a differential amplifier that outputs a differential signalfrom an inversion output terminal and a non-inversion output terminal, afirst feedback resistor that is connected between the non-inversionoutput terminal and inversion input terminal of the differentialamplifier, a second feedback resistor that is connected between theinversion output terminal and non-inversion input terminal of thedifferential amplifier and that has the same resistance value as thefirst feedback resistor, a first input resistor that is inserted betweenthe inversion input terminal of the operational amplifier and a firstinput terminal of the variable gain amplifier circuit and whoseresistance value is variable, a second input resistor that is insertedbetween the non-inversion input terminal of the operational amplifierand a second input terminal of the variable gain amplifier and whoseresistance value is variable, and an adjustment resistor that isconnected between the inversion input terminal of the operationalamplifier and the non-inversion input terminal or the operationalamplifier. The resistance values of the first and second input resistorsare controlled to be the same. The resistance value of the adjustmentresistor is controlled in such a way as to maintain a constant parallelresistance value in the parallel connection between a resistor whoseresistance value is the same as the first and second input resistorsafter the change and a resistor whose resistance value is ½ of that ofthe adjustment resistor against the resistance value change whenchanging resistance values of the first and second input resistors.

According to this configuration, even when changing one resistance valueof the input resistor and the feedback resistor in order to change acircuit, changes in the amount of feedback of the operational amplifiercan be suppressed to maintain it constantly. Therefore, a differentialamplifier whose phase characteristic does not vary even when changingthe gain of the circuit can be provided.

A filter circuit in another aspect of the present invention can vary again of the filter circuit. The variable gain amplifier circuitcomprises a fully-differential RC active filter that is composed of anoperational amplifier, a resistor and a capacitor and that comprises twopositive and negative input terminals and two positive and negativeoutput terminals, a positive-side input resistor that is insertedbetween a positive input terminal of the RC active filter and anon-inversion input terminal of the operational amplifier and whoseresistance value is variable, a negative-side input resistor that isinserted between a negative input terminal of the RC active filter andan inversion input terminal of the operational amplifier and whoseresistance value is variable, and an adjustment resistor that isconnected between the inversion input terminal and the non-inversioninput terminal of the operational amplifier. The resistance values ofthe positive-side and negative-side input resistors are controlled to bethe same. A resistance value of the adjustment resistor is controlled insuch a way as to maintain constant against the resistance value change aparallel resistance value between a resistor whose resistance value isthe same as the positive-side and negative-side input resistors afterthe change and a resistor whose resistance value is ½ of that of theadjustment resistor when changing resistance values of the positive-sideand negative-side input resistors.

According to this configuration, even when changing one resistance valueof the input resistor and the feedback resistor in order to change acircuit, changes in the amount of feedback of the operational amplifiercan be suppressed to maintain it constantly. Therefore, a filter circuitwhose phase characteristic does not vary even when changing the gain ofthe circuit can be provided.

As described above, according to any aspect of the present invention, avariable gain amplifier circuit in which the amount of feedback does notvary even when the gain changes can be provided for use in variouscircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the following detaileddescription when the accompanying drawings are referenced.

FIG. 1 is one example of a variable gain amplifier circuit whose gaincan be changed.

FIG. 2 is a graph showing the change of the frequency and phasecharacteristics of the circuit shown in FIG. 1 when its gain is changed.

FIG. 3 shows the first configuration example of the variable gainamplifier circuit implementing the present invention.

FIG. 4 shows the change of the frequency and phase characteristics ofthe circuit shown in FIG. 3 when changing the value of the inputresistor and also adjusting the value of the adjustment resistor.

FIG. 5A shows the simulation result of the variable gain amplifiercircuit (No. 1).

FIG. 5B shows the simulation result of the variable gain amplifiercircuit (No. 2).

FIG. 6 shows the detailed configuration example of the variable gainamplifier circuit shown in FIG. 3.

FIG. 7 shows the first example used of the variable gain amplifiercircuit shown in FIG. 3.

FIG. 8 shows the second example used of the variable gain amplifiercircuit shown in FIG. 3.

FIG. 9A shows the first example of the filter circuit with a variablegain function.

FIG. 9B is a graph showing the change of the frequency characteristiccaused when changing the gain of the filter circuit shown in FIG. 9A.

FIG. 10A shows an example configuration of the filter circuitimplementing the present invention.

FIG. 10B shows the change of the frequency characteristic of the circuitshown in FIG. 10A when changing the value of the input resistor and alsoadjusting the value of the adjustment resistor.

FIG. 11A shows the simulation result of the filter circuit (No. 1).

FIG. 11B shows the simulation result of the filter circuit (No. 2).

FIG. 12 shows the second configuration example of the variable gainamplifier circuit implementing the present invention.

FIG. 13 shows the third configuration example of the variable gainamplifier circuit implementing the present invention.

FIG. 14 shows the fourth configuration example of the variable gainamplifier circuit implementing the present invention.

FIG. 15 shows the second example of the filter circuit with a variablegain function.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are described inreference to the drawings.

Firstly, FIG. 3 is described. FIG. 3 shows the first configurationexample of the variable gain amplifier circuit implementing the presentinvention.

In FIG. 3, the non-inversion input terminal of the operational amplifierOP1 is connected to the ground (or reference potential). A feedbackresistor R0 is connected between the output terminal and the inversioninput terminal of the operational amplifier.

An input resistor R1 is connected between the input terminal IN of thisvariable gain amplifier circuit and the inversion input terminal of theoperational amplifier. The resistance value of this input resistor R1 isvariable.

Furthermore, an adjustment resistor Rs is connected to the inversioninput terminal of the operational amplifier and the ground (or referencepotential) of the circuit. The resistance value of this adjustmentresistor is also variable.

The resistance value of this adjustment resistor Rs is controlled insuch a way as to maintain constant the ratio between the resistancevalues of the combined resistance value in its parallel connection withthe input resistor R1 and the feedback resistor R0 when changing theresistance value of the input resistor R1 or the feedback resistor R0.However, in the circuit shown in FIG. 3, since the resistance value ofthe feedback resistor R0 is fixed, the resistance value of theadjustment resistor Rs is controlled in such away as to maintainconstant the combined resistance value in its parallel connection withthe input resistor R1 when changing the resistance value of the inputresistor R1.

In the circuit shown in FIG. 3, the gain G of the circuit is as follows,as with the circuit shown in FIG. 1 (here, no inversion of a signal isconsidered):G=R0/R1Therefore, when changing the value of the input resistor R1, which isvariable, the gain of the circuit varies.

In this case, the adjustment resistor Rs is adjusted in such away as tomaintain R1//Rs (the combined resistance value in the parallelconnection between the input resistor R1 and the adjustment resistor Rs)constant against changes in the input resistor R1 that change thecircuit gain.

FIG. 4 is a graph showing change in the frequency and phasecharacteristics of the circuit shown in FIG. 3 that is caused whenchanging the gain of the circuit by changing the value of the inputresistor R1 and also adjusting the value of the adjustment resistor Rsas described above.

In the circuit shown in FIG. 3, the amount of feedback of theoperational amplifier OP1 to the inversion input terminal is shown asfollows:V₀×(R1//Rs)/{(R1//Rs)+R0}In this case, when adjusting the value of the adjustment resistor Rs insuch a way as to maintain R1//Rs constant against changes in the inputresistor R1, the amount of feedback becomes fixed regardless of thechanges in the input resistor R1. Thus, as shown in FIG. 4, even whenchanging the gain of the circuit by changing the value of the inputresistor R1, changes in the phase characteristic can be suppressed.

Here, FIGS. 5A and 5B are described. FIGS. 5A and 5B show the simulationresult of the variable gain amplifier circuit. FIGS. 5A and 5B are thesimulation results of the circuits shown in FIGS. 1 and 3, respectively.Curves shown in the respective upper sections of FIGS. 5A and 5B showthe frequency characteristic and those in the respective lower sectionsshow the phase characteristic.

In the simulation shown in FIGS. 5A and 5B it is assumed that the gainof the operational amplifier On is 1,000 times and the feedback resistorR0 is 10 kΩ. The solid curve shows the result in a case in which theinput resistor R1 is 1.0 kΩ (in this case, the adjustment resistor Rs is∞), the broken curve is the result in a case in which the input resistorR1 is 3.3 kΩ (in this case, the adjustment resistor Rs is 1.43 kΩ) andthe one-point chain curve is the result in a casein which the inputresistor R1 is 10 kΩ (in this case, the adjustment resistor Rs is 1.11kΩ). However, the phase characteristic curve shown in FIG. 5B isobtained by combining these three curves.

Comparing FIGS. 5A and 5B shows that, when changing the gain in thecircuit shown in FIG. 1, the phase characteristic varies and the amountof slewing of the phase varies in the higher frequency area, while thephase characteristic does not vary in the circuit shown in FIG. 3 evenwhen changing the gain.

Next, FIG. 6 is described. FIG. 6 shows a detailed configuration exampleof the variable gain amplifier circuit shown in FIG. 3.

In FIG. 6, an operational amplifier OP1, a feedback resistor R0, aninput resistor R1 and an adjustment resistor Rs are formed on a singlesemiconductor substrate of a semiconductor device 1 to form a variablegain amplifier circuit. This semiconductor device 1 can be, for example,an analog integrated circuit (IC) for high-frequency signal processingand is installed in electronic equipment 2 such as a radio signaltransmitter, receiver, or the like.

In the circuit shown in FIG. 6, the configuration of the input resistorR1 and adjustment resistor Rs differs from that of FIG. 1. Specifically,in the circuit shown in FIG. 6, the input resistor R1 is formed byconnecting the serial connection of a resistor R1 ₁ and an (electric)field effect transistor (FET) M1 ₁, that of a resistor R1 ₂ and an FETM1 ₂, . . . , and that of a resistor R1 _(n) and an FET M1 _(n) inparallel, and the adjustment resistor Rs is formed by connecting theserial connection of a resistor Rs₁ and an (electric) field effecttransistor (FET) Ms₁, that of a resistor Rs₂ and an FET Ms₂, . . . , andthat of a resistor Rs_(n) and an FET Ms_(n) in parallel.

In this case, the resistance values of the resistors R1 ₁, R1 ₂, . . .and Rs_(n) are different from each other. The resistors Rs₁, Rs₂, . . .and Rs_(n) are related to the resistors R1 ₁, R1 ₂, . . . and R1 _(n),respectively, one to one, and each resistance value is set as follows:R1₁ //Rs ₁ =R1 ₂ //Rs ₂ . . . R1_(n) //Rs _(n)Specifically, each resistance value of the resistors Rs₁, Rs₂, . . . andRs_(n) is set such that all of the combined resistance values inparallel connection of each combination of respective related resistorsR1 ₁, R1 ₂, . . . and R1 _(n) and resistors Rs₁, Rs₂, . . . and Rs_(n)become the same.

In FIG. 6, a prescribed voltage sufficiently higher than the ground(reference potential) of the circuit is applied to one of terminals S₁,S₂, . . . and S_(n). In this case, when applying the prescribed voltageto, for example, the terminal S₁, FET M1 ₁ and MS₁ are switched on.Then, the resistor R1 ₁ is selected as the input resistor R1 and isconnected between the input terminal IN of the variable gain amplifierand the inversion input terminal of the operational amplifier OP1. Then,the resistor Rs₁ corresponding to the resistor R1 ₁ is selected as theadjustment resistor Rs and is connected between the inversion inputterminal of the operational amplifier OP1 and the ground (or referencepotential) of the circuit. Therefore, by switching one of the terminalsS₁, S₂, . . . and S_(n) to which the prescribed voltage is applied, thevalue of the input resistor R1 varies in order to change the gain of thecircuit shown in FIG. 6. In this case, the resistance value of theadjustment resistor Rs is controlled to maintain constant againstchanges in the value of the input resistor R1 the combined resistancevalue in the parallel connection between the adjustment resistor Rs andinput resistor R1 in order to suppress changes in the phasecharacteristic of the circuit.

The first example used of the variable gain amplifier circuit shown inFIG. 3 is shown in FIG. 7. In the circuit shown in FIG. 7, thesemiconductor device 1 further comprises a register 3. When an externaldevice (for example, a microprocessor) writes data into the register 3,the prescribed voltage is applied to one of the terminals S₁, S₂, . . .and S_(n), corresponding to the written data. The value of the inputresistor R1 is set as this result in order to change the gain of thecircuit. Simultaneously, the resistance value of the adjustment resistorRs is controlled according to the value of the input resistor R1 inorder to suppress the change of the phase characteristic of the circuit.

FIG. 8 shows the second example used of the variable gain amplifiercircuit shown in FIG. 3 and shows the circuit it is used in, anautomatic gain control (AGC) circuit. In the circuit shown in FIG. 8,the semiconductor device 1 further comprises a signal level detectorcircuit 4 and a control circuit 5.

In FIG. 8, the signal level detector circuit 4 detects the signal levelof an output signal from the variable gain amplifier circuit. Thecontrol circuit 5 performs controls in order to change the value of theinput resistor R1 on the basis of the signal level detected by thesignal level detector circuit 4 and also controls the resistance valueof the adjustment resistor Rs according to the value of the inputresistor R1, as described above. By performing this control, the gain ofthe circuit is caused to vary according to the signal level of theoutput signal from the variable gain amplifier circuit and also iscaused to suppress changes in the phase characteristic against thischange in gain.

Next, the filter circuit employing the variable gain amplifier circuitof the present invention is described.

Firstly, FIG. 9A is described. FIG. 9A is a circuit diagram showing oneexample of the filter circuit with a variable gain function.

The circuit shown in FIG. 9A is a first-order low-pass filter (LPF)circuit in which a capacitor C is connected to the feedback resistor R0in the variable gain amplifier circuit shown in FIG. 1.

FIG. 9B is a graph showing the change of the frequency characteristiccaused when changing the DC gain of the filter circuit shown in FIG. 9A.As seen from this graph, when reducing the DC gain of the circuit shownin FIG. 1 by increasing the value of the input resistor R1, the phasecharacteristic of the variable gain amplifier circuit also varies, whichchanges the cut-off frequency and Q value of the filter. This change inthe cut-off frequency and Q value is also caused by the change in theamount of feedback from the operational amplifier OP1 to the inversioninput terminal due to the increase in the value of the resistor R1.

Next, FIG. 10A is described. FIG. 10A shows an example configuration ofthe filter circuit implementing the present invention.

The circuit shown in FIG. 10A is a first-order low-pass filter (LPF)circuit in which a capacitor C is connected to the feedback resistor R0in the variable gain amplifier circuit shown in FIG. 3. It can also beconsidered that the circuit shown in FIG. 10A is obtained by providingthe adjustment resistor Rs between the inversion input terminal of theoperational amplifier OP1 in the circuit shown in FIG. 9A and thereference potential.

In the circuit shown in FIG. 10A, the DC gain G of the circuit is asfollows, as with the circuit of FIG. 9A (in this case, no inversion of asignal is considered):G=R0/R1Therefore, when changing the value of the input resistor R1, which isvariable, the DC gain of the circuit varies.

In this case, the adjustment resistor Rs is adjusted in such a way as tomaintain R1//Rs constant against changes in the input resistor R1 thathave the purpose of modifying the DC gain of the circuit.

The change of the frequency characteristic of the circuit shown in FIG.10A, caused when changing the DC gain of the circuit by changing thevalue of the input resistor R1, and also adjusting the value of theadjustment resistor Rs as described above, are shown in FIG. 10B.

In the circuit shown in FIG. 10A as well, the amount of feedback of theoperational amplifier to the inversion input terminal as shown asfollows:V₀×(R1//Rs)/{(R1//Rs)+R0}In this case, when adjusting the value of the adjustment resistor Rs insuch a way as to maintain R1//Rs constant against changes in the valueof the input resistor R1, the amount of feedback becomes constantregardless of changes in the input resistor R1. Thus, as shown in FIG.10B, even when changing the DC gain of the circuit by changing the valueof the input resistor R1, the cut-off frequency and Q value do not vary.

Here, FIGS. 11A and 11B are described. FIGS. 11A and 11B show thesimulation results of the filter circuit. FIG. 11A shows the simulationresult obtained when the variable gain filter circuit shown in FIG. 9Ais employed in the final 2 pole stage of an active LPF circuit with afifth-order Butterworth characteristic. FIG. 11B shows the simulationresult obtained when the variable gain filter circuit shown in FIG. 11Bis employed in the final 2 pole stage of an active LPF circuit with afifth-order Butterworth characteristic.

FIGS. 11A and 11B are compared. The curve shown in FIG. 11A shows thatthe cut-off frequency of the LPF varies according to the change of theDC gain, and the shape of the curve in the neighborhood of the cut-offfrequency varies according to the change in the DC gain (specifically,the Q value of the filter varies). The curve shown in FIG. 11B showsthat the cut-off frequency of the LPF does not vary even when changingthe DC gain, and also that the shape of the curve in the neighborhood ofthe cut-off frequency does not vary (specifically, the Q value of thefilter does not vary).

It is clear that the filter circuit shown in FIG. 10A can be formed byconnecting the capacitor C to the feedback resistor R0 in the variablegain amplifier circuit whose detailed configuration is shown in FIG. 6,in parallel. This capacitor can also be formed on the semiconductorsubstrate of the semiconductor device 1.

Next, FIG. 12 is described. FIG. 12 shows the second configurationexample of the variable gain amplifier circuit implementing the presentinvention.

In FIG. 12, the non-inversion input terminal of the operationalamplifier OP1 is connected to the ground (or reference potential) of thecircuit. A feedback resistor R0 is connected between the output terminaland the inversion input terminal of the operational amplifier OP1. Inthis case, unlike the first example shown in FIG. 3, the resistancevalue of this feedback resistor R0 is variable.

An input resistor R1 is connected between the input terminal IN of thisvariable gain amplifier circuit and the inversion input terminal of theoperational amplifier. In this case, unlike the first example shown inFIG. 3, the resistance value of this input resistor R1 is fixed.

Furthermore, the adjustment resistor Rs, which features the presentinvention, is connected between the inversion input terminal of theoperational amplifier OP1 and the ground (or reference potential) of thecircuit. The resistance value of this adjustment resistor Rs isvariable.

The resistance value of this adjustment resistor Rs is controlled insuch a way as to maintain constant against the resistance value changethe ratio between the combined resistance value in its parallelconnection with the input resistor R1 and the resistance value of thefeedback resistor R0 when changing one resistance value of the inputresistor R1 and the feedback resistor R0. In this circuit shown in FIG.12, since the resistance value of the input resistor R1 is fixed, theresistance value of the adjustment resistor Rs is controlled in such away as to maintain constant against the resistance value change theratio between the combined resistance value in its parallel connectionwith the input resistor R1 and the resistance value of the feedbackresistor R0 when changing the resistance value of the feedback resistorR0.

In the circuit shown in FIG. 12, like the circuit shown in FIG. 3, thegain G of the circuit is as follows (in this case, no inversion ofsignal is considered):G=R0/R1Therefore, when changing the value of the feedback resistor R0, which isvariable, the gain of the circuit varies.

In this case, the adjustment resistor Rs is adjusted in such a way as tomaintain constant, against changes in the feedback resistor R0 that havethe purpose of modifying the gain of the circuit, the ratio betweenR1//Rs and the resistance value of the feedback resistor R0.

In the circuit shown in FIG. 12, the amount of feedback of theoperational amplifier to the inversion input terminal is as follows, aswith the circuit shown in FIG. 3:V₀×(R1//Rs)/{(R1//Rs)+R0}In this case, when the value of the adjustment resistor Rs is adjustedin such a way as to maintain constant against changes in the feedbackresistor R0 the ratio between R1//Rs and the resistance value of thefeedback resistor R0, the amount of feedback becomes constant regardlessof changes in the feedback resistor R0. Thus, even when changing thegain of the circuit by changing the value of the feedback resistor R0,changes in the phase characteristic can be suppressed.

Next, FIG. 13 is described. FIG. 13 shows the third configurationexample of the variable gain amplifier circuit implementing the presentinvention. This is a differential amplifier circuit.

In FIG. 13, the operational amplifier OP1 is a differential output typeoperational amplifier that outputs a differential signal from itsinversion output terminal (OUTM side) and non-inversion output terminal(OUTP side). A feedback resistor R0 with the same resistance value isconnected between the non-inversion output terminal and inversion inputterminal of this operational amplifier and between its inversion outputterminal and non-inversion input terminal. An input resistor R1 with thesame resistance value is connected between the inversion side inputterminal INM of this variable gain amplifier circuit and the inversioninput terminal of the operational amplifier OP1 and between thenon-inversion side input terminal of the variable gain amplifier circuitand the non-inversion input terminal of the operational amplifier OP1.The resistance values of these two input resistors R1 are variable.However, when changing one of them, the other is controlled in such away that it may become the same as that of the changed one after thechange.

Furthermore, the adjustment resistor is connected between the inversioninput terminal and non-inversion input terminal of the operationalamplifier. This adjustment resistor is variable and its resistance valueis 2×Rs.

The resistance value of this adjustment resistor is controlled in such away as to maintain constant against the resistance value change theratio between the combined resistance value in the parallel connectionbetween a resistor Rs whose resistance value is ½ of the adjustmentresistor and the input resistor R1 and the resistance value of thefeedback resistor R0 when changing one resistance value of the inputresistor R1 and the feedback resistor R0. In the circuit shown in FIG.13, since the resistance value of the feedback resistor R0 is fixed, theresistance value of the adjustment resistor is controlled in such a wayas to maintain constant against the resistance value change the combinedresistance value in the parallel connection between the resistor Rswhose resistance value is ½ of the adjustment resistor and the inputresistor R1 when changing the resistance value of the input resistor R1.

In the circuit shown in FIG. 13 as well, the gain G of the circuit is asfollows, as in the circuit shown in FIG. 3 (in this case, no inversionof signal is considered):G=R0/R1Therefore, when changing the value of the input resistor R1, which isvariable, the gain of the circuit varies.

In this case, the adjustment resistor is adjusted in such a way as tomaintain R1//Rs (in this case, the resistor whose resistance value is ½of the adjustment resistor) constant against changes in the inputresistor R1 whose purpose is to modify the gain of the circuit.

In the circuit shown in FIG. 13, as with the circuit shown in FIG. 3,the amount of feedback of the operational amplifier OP1 to the two inputterminals is as follows.V₀×(R1//Rs)/{(R1//Rs)+R0}

In this case, when the value of the adjustment resistor is adjusted insuch a way as to maintain R1//Rs constant against changes in the valueof the input resistor R1, the amount of feedback becomes constantregardless of changes in the feedback resistor R0. Thus, even whenchanging the gain of the circuit by changing the value of the inputresistor R1, changes in its phase characteristic can be suppressed.

In the circuit shown in FIG. 14, like FIG. 12, the feedback resistor R0and the input resistor R1 can also be variable and fixed, respectively,instead of being the fixed feedback resistor R0 and the variable inputresistor R1 shown in FIG. 13; further, the resistance value of theadjustment resistor can also be controlled in such a way as to maintainconstant against the resistance value change the ratio between thecombined resistance value in the parallel connection between theresistor Rs, whose resistance value is ½ of the adjustment resistor andthe input resistor R1, and the resistance value of the feedback resistorR0. Thus too, even when changing the gain of the circuit by changing thevalue of the feedback resistor R0, the change of its phasecharacteristic can be suppressed.

When the filter circuit is composed in such a way as to change the DCgain of the circuit by changing the resistance value of the inputresistor by providing the circuit shown in FIG. 13 with a capacitor (forexample, providing the feedback resistor with a capacitor, as which isshown in FIG. 15), the change of its phase characteristic can also besuppressed by controlling the resistance value of the adjustmentresistor in such a way as to maintain the parallel resistance valuebetween a resistor whose resistance value is after the change and theadjustment resistor when changing the resistance value of the inputresistor.

Although the preferred embodiments of the present invention have so farbeen described, the present invention is not limited to theabove-described preferred embodiments and can be improved/modified aslong as the improvements/modifications do not cause it to deviate fromthe main subject of the present invention.

1. A filter circuit which can vary its gain, comprising: an RC activefilter composed of an operational amplifier, a resistor and a capacitor;a variable input resistor that is inserted between an input terminal ofthe RC active filter and an inversion input terminal of the operationalamplifier; and a variable adjustment resistor that is connected betweenthe inversion input terminal of the operational amplifier and areference potential of the filter circuit, wherein a resistance value ofthe adjustment resistor is controlled so as to maintain a parallelresistance value between the variable adjustment resistor and thevariable input resistor constant when changing a resistance value of theinput resistor.
 2. A filter circuit according to claim 1, wherein thevariable input resistor is coupled to the inversion input terminal ofthe operational amplifier directly.
 3. A filter circuit according toclaim 1, wherein the resistor of the RC active filter is coupled to oneterminal of the variable input resistor where an input terminal of theRC active filter is not coupled.
 4. A filter circuit according to claim1, wherein a gain which is fed back to the inversion input terminal ofthe operational amplifier is substantially constant.